The present invention relates to a friction stir welding method.
A friction stir welding method is a technique in which a round shaft (called a rotary tool) is rotated and inserted into the welding region of members to be welded, and the rotating rotary tool is moved along the welding line, thereby heating, mobilizing and plasticising the material in the welding region, to achieve a solid-phase welding of the members. The rotary tool is comprised of a large-diameter portion and the small-diameter portion. The small-diameter portion is inserted into the joint between the members to be welded so that the end surface of the large-diameter portion comes into contact with the aforementioned members. A screw thread is formed on the small-diameter portion.
Moreover, to facilitate the friction stir welding, a raised convex portion is provided on the two members to be welded in the region where the rotary tool is to be inserted, and the small-diameter portion of the rotary tool is inserted into the butted region, so that the large-diameter portion of the rotary-tool is inserted into the raised portions. The gap between the two members is filled using the metal of the raised portions as the source. The raised portion may be formed only on one of the members. This technique is used in a case of friction stir welding members made of extruded materials, at a region where the extruded directions are orthogonalized.
This technique is disclosed in Japanese Patent Laid-open Publication No. 11-90655 (U.S. Pat. No. 6,050,474).
FIGS. 8(A) and 8(B) are longitudinal cross-sectional views of a friction stir welded region where a member M1 provided with a raised portion P1 is abutted against a member M2 not provided with a raised portion. FIG. 8(A) shows the condition where a rotary tool 50 is inserted into the butted region between the two members M1, M2. FIG. 8(B) is a longitudinal cross-sectional view showing the region after welding, and the hatching indicates the stirred area of the weld. The indication of the stirred area is an exemplification. The rotary tool 50 for friction stir welding is comprised of a large-diameter portion 51 and a small-diameter portion 52 at the leading end thereof. When performing friction stir welding, the small-diameter portion 52 is inserted into the butted region to a level where a part of the large-diameter portion 51 is inserted into the raised portion P1. The leading end of the small-diameter portion 52 is inserted into a projecting segment P2 on which the rear surface of the member M2 rests. The lower end of the large-diameter portion 51 is positioned so as to come into contact with the upper surface of the member M2. When welding is performed, continuous metal oxide A is formed near the surface of the welded region on the side of the member M2 having no raised portion, as indicated in FIG. 8(B). It is considered that this metal oxide A appears at the surface of the member being enwound and not diffused. Also, the surface portion which overlaps with the projecting segment P2 and the member M2 tends to form a notch B extending towards the upper surface side (large-diameter portion Ti side of the rotary tool T). It is considered that the continuous metal oxide A or the notch B deteriorates the strength of the weld.
The object of the present invention is to provide a friction stir welding method that could restrict the occurrence of defects.
As a result of various examinations, it has been formed that the pressure of metal fluidized by the rotation of the rotary tool is not uniform throughout the projected area in the axis direction of the rotary tool, but is different depending on the axial position. In this regard, the position with high pressure is defined by the direction of rotation and the direction of movement of the rotary tool. It is considered that this non-uniformity leads to occurrence of the continuous metal oxide A or the notch B.
The present invention is characterized by the fact that a member with no raised portion is placed in the position where high pressure is provided, which position with higher pressure of the fluidized metal is defined by the direction of rotation and direction of movement of the rotary tool.